The Heat-Affected Zone (HAZ) is one of the most critical aspects of welding metallurgy. It's the area of base metal that is not melted but has undergone significant changes in its microstructure due to exposure to high temperatures during welding. The HAZ can affect the mechanical properties of the metal, such as its hardness, toughness, and susceptibility to cracking. Controlling the HAZ is crucial in maintaining the integrity of the weld joint and the overall structure.

1. What is the Heat-Affected Zone (HAZ)?

The HAZ refers to the portion of the base material adjacent to the weld that has experienced thermal cycles (heating and cooling) intense enough to alter its microstructure, but not enough to melt it. While the weld pool itself forms the fusion zone (FZ), the HAZ surrounds this area and is divided into various temperature gradients, each affecting the material differently.

In many materials, especially carbon steels, stainless steels, and alloy steels, the HAZ is a critical factor in weld performance. The thermal history that the HAZ experiences during welding can induce hardness, brittleness, grain growth, and potential cracking if not carefully managed.

2. Metallurgical Changes in the HAZ

The changes that occur in the HAZ depend on several factors, including the material composition, the welding process, and the cooling rate. The HAZ can be broken down into three key subzones:

  • Coarse Grain Heat-Affected Zone (CGHAZ): Closest to the fusion zone, the CGHAZ experiences the highest temperatures just below the melting point of the base material. In steel, this causes grain growth and significant microstructural changes. Coarser grains result in reduced toughness, making the material more susceptible to cracking.

  • Fine Grain Heat-Affected Zone (FGHAZ): As you move away from the fusion zone, the metal experiences lower temperatures, leading to finer grain structures. Finer grains improve toughness and ductility compared to the coarse-grain zone.

  • Intercritical and Subcritical HAZ: These regions are farthest from the fusion zone and experience temperatures below the transformation point. The subcritical HAZ undergoes tempering, while the intercritical zone sees partial phase transformations. In steels, this area might include a mix of ferrite and pearlite or other phases, depending on the material.

In materials like aluminum alloys, the HAZ can cause precipitate dissolution and over-aging, reducing the material’s strength, which can be problematic in aerospace applications.

3. Effect of Welding Parameters on the HAZ

The extent and properties of the HAZ are highly dependent on the welding process parameters:

  • Heat Input: This is a critical factor influencing the size and properties of the HAZ. Heat input is determined by the welding process, current, voltage, and travel speed. A high heat input increases the size of the HAZ and can lead to grain coarsening and softening of the base metal in steels, increasing the risk of cracking.

    Formula: Heat Input (kJ/mm) = (Voltage * Current * 60) / (1000 * Travel Speed)

  • Cooling Rate: The cooling rate after welding has a significant impact on the microstructural evolution of the HAZ. Rapid cooling in steels can lead to the formation of martensite, a hard but brittle phase, making the weld joint more prone to cracking. Controlled cooling, such as post-weld heat treatment (PWHT), can relieve residual stresses and temper martensitic structures, enhancing toughness.

  • Welding Technique: The use of multi-pass welding (especially in thicker materials) can alter the thermal cycles experienced by the HAZ, with subsequent passes reheating and tempering previously welded areas. This can improve the toughness of the HAZ.

4. Common Problems Associated with the HAZ

  • HAZ Cracking: Cracking in the HAZ is a common issue, especially in high-strength steels or thick sections. Hydrogen-induced cracking (HIC) or cold cracking often occurs due to the combination of a high hardness HAZ, residual stresses, and hydrogen absorption during welding.

  • Brittleness and Hardness: If the HAZ experiences too much grain coarsening or forms martensitic structures in steels, it can become excessively hard and brittle, increasing the risk of brittle fracture under stress.

  • Softening in Aluminum: In heat-treated aluminum alloys, such as 6061, the HAZ can experience precipitate dissolution, leading to softening. The strength of the aluminum alloy is significantly reduced in the HAZ compared to the parent material.

5. Controlling the HAZ

To ensure optimal weld performance and minimize problems in the HAZ, several control methods are used:

  • Preheating: Preheating the base material before welding helps reduce the cooling rate, minimizing the risk of HAZ hardening and cracking, especially in carbon steels. Preheating temperatures depend on the material but can range from 150°C to 300°C.

  • Post-Weld Heat Treatment (PWHT): PWHT is a thermal process applied after welding to relieve residual stresses and improve toughness in the HAZ. In steels, PWHT reduces the hardness of martensite and improves ductility. The process typically involves heating the welded assembly to a temperature just below the transformation range and holding it for a specified time.

  • Low-Hydrogen Electrodes: Using low-hydrogen electrodes (such as E7018 for stick welding) or properly controlled shielding gases reduces hydrogen content in the weld, minimizing the risk of hydrogen-induced cracking in the HAZ.

  • Optimizing Heat Input: By using controlled heat input processes, such as pulsed MIG or TIG welding, welders can reduce the size of the HAZ and minimize grain growth. Pulsed techniques deliver high energy only during certain parts of the welding cycle, which controls the amount of heat absorbed by the base material.

6. Modern Techniques to Minimize HAZ Damage

Recent advancements in welding technology offer new ways to reduce the impact of the HAZ:

  • Laser Welding: Laser welding provides a highly focused heat source, minimizing heat input and significantly reducing the size of the HAZ. This technique is ideal for materials like stainless steel and titanium.

  • Electron Beam Welding: Like laser welding, electron beam welding delivers high energy density, reducing the HAZ and associated metallurgical changes.

Conclusion

The Heat-Affected Zone is a complex but critical aspect of welding that can significantly impact the performance of welded joints. Understanding how metallurgical changes in the HAZ occur and how to control them through process parameters, preheating, and post-weld treatments is essential for achieving strong, reliable welds. Proper control of the HAZ ensures longevity, reduces cracking risks, and optimizes the mechanical properties of the welded joint.

For more insights on welding techniques and advanced equipment, contact Quantum Machinery Group at Sales@WeldingTablesAndFixtures.com or call (704) 703-9400.

Measuring Wheel

Measuring wheel also called mechanical ranging car or digital distance measuring wheel.it is widely used to measuring distance in professional surveying and mapping work,road engineering, landscape planning and design,golf course and lanes ect.




The instruction of digital measuring wheel

Please read this instruction before you use the product

Thanks you for choosing our product. This instrument is a new length measuring instrument, it is easy to use,stable and reliable.

The main features:

You could convert the unit between metric and foot; The data could be stored and extracted; The touch switch of the digital measuring wheel is anti-water; The handle which could adjust the height of the instrument is easy to apply. The wheel wrapped with rubber could help to avoid slipping so that the measuring data is more precise.

1.The main parts (From up to down):

Handle

Data display panel

Battery cover

Handle lock

Measuring wheel

Kickstand

2. Technical data:

A. It could measure up to 99999.9m/ft.

B. Accuracy:+/- 0.3%

C: Minimal display :0.1m

D: Power supply 3V (2xAAA LR003)

E: Suitable temperature for use : -10c°~45c°

3. Button and Operation


CLR: Clearance

SM : Store the data

ON/OFF: Switch

Rm: Extra the data

M/ft: Conversion between metric and foot

Data display panel


Power supply

The stored data

The displayed data

Metric

Foot

1. ON/OFF : Switch.

2. m/ft: it means the conversion between metric and foot. If you select (m), the displayed data is based on metric. If you select (ft) ,the displayed data is based on foot.

3. SM: Store the data. You could save the data that shows on the display panel by pressing this button. It could store 5 records that respectively show on M1, M2,M3, M4, M5.( Refer to Table 1)

Before storing After storing Full of the memory

(Table 1)

4. RM: Extract the stored data from SM. For example, if you have already stored the measuring data 5M as M1, 10M as M2. But now the measuring data is 120.7M, you could press the bottom RM once then the digital display panel will shows the the data of M1. The top right of the display panel shows [R". The data of the digital display panel will return to the present data after two seconds. ( Refer to Table 2). If you press the RM twice, the data of the M2 will show on the display panel, the [R" also is on the top right. The data of the digital display panel will return to the present data again after two seconds.

Present data Press the RM Once After 2 seconds

(Table 2)

Present data Press the RM twice After 2 seconds

(Table 3)

5. CLR: Clearance. Once you press this button, the present data will be deleted.

4.Precautions for use

Please open the handle lock to adjust the suitable height of this instrument at first and then close it before using the instrument. Put away the kickstand and turn on the power. Put the wheel on the road to be measured when the data is 0.0m on the digital display panel. Please press the bottom CLR if you want to delete the present data.

1. Please go straight if you want to measure a straight line to ensure the accuracy of the measurement.

2. Do not measure uneven surfaces because it will reduce the accuracy.

3. Do not measure in the rain for a long time.

4. Do not use the instrument under the high temperature environment or keep this product in the high temperature place.

5. Please open the battery cover by using the cross-head screwdriver and change the battery when the digital display panel is dark.

6. Do not deal with the used batteries by yourself, please send them to the government designated place where to recycle them.

7. Please remove the battery and clean the instrument by use the mild detergent to prevent rust if you haven`t used it for a long time. Then please pack it well and save it in the rough place.

8. Do not use benzene solution to clean the instrument, otherwise its surface will be corroded.

9. Do not use this the instrument as a toy for child for avoiding unnecessary damage.

Measuring Wheel,Measuring Wheel Walking,Wheel Measuring Tool

Wintape Measuring Tape Company , https://www.wintapetape.com